In the world I'm making I want to have very large creatures that will later be used in organic architecture as housing for the human equivalent in this world. In order to make this world realistic I am only going to be pushing the creatures to the upper limit of the Square-Cube Law. This has been discussed at length on this site and there are quite a few examples of the size limits of different body types and activity levels.

In order to have the city sized creatures I want, I plan on shrinking my humanoids to the smallest size that can still be mistaken for full size. Let me clarify a little better. How small can I make my humanoids before we start getting weird physics and biology? I don't want them to be carrying water in their hands and throwing it like water balloons and they need to have a very similar biology to full sized humans (no open circulatory system, they have to be warm blooded, and they can't be able to jump 100x their body length). I know someone already asked about microscopic humans but I want to find the sweet spot between small size and normal physics (yes I know this question is about use of fire but I couldn't find the one that asked about anatomy).

You can ignore comparisons to other plants and animals. I will be adjusting their size as well and most likely creatures like mice will be replaced with relatively larger bugs.

$\begingroup$I think one of the major limiters would be a functioning human brain. It generally looks like intelligence relates closely to brain body ratio. Over all brain mass is a factor though so you may end up making humans dumber shrinking them significantly$\endgroup$
– TathelOct 13 '16 at 0:36

2 Answers
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Homo floresiensis was a subspecies of hominid (sometimes nicknamed "hobbits") that was around 3-4 feet tall. While not much is known of their culture and lifestyle, they did make tools, and given that modern dwarfism can result in humans that size or smaller, there is no reason to believe that they would be less intelligent.

If you want to go smaller, you probably can get fairly small without running into serious square-cube law issues. Capuchin monkeys are quite small, but water will still behave effectively the same for them as it does for us.

Once you get down to mouse-size you might start running into problems with heat loss, so humans that small might be more "round" to minimize surface area. You might not want that.

The square cube law will make people proportionally stronger and more agile at small sizes, but depending on what you mean by "weird physics" it might not be too bad. As a general rule, if body shape remains the same you can presume that speed and jumping height will remain more or less unchanged - if you can jump 2 feet and run at 10 mph, you shrunken down to 2 foot tall can also jump 2 feet and run at 10 mph. Is it a serious issue if everyone in the world can move like a professional sports player (and a professional sports player can move like a superhero)?

$\begingroup$"if you can jump 2 feet and run at 10 mph, you shrunken down to 2 foot tall can also jump 2 feet and run at 10 mph" I think this is wrong, yes a small person is stronger for his size but still less strong. If you can lift 60% of your weight, half-you can lift 100% of his weight you still lift a heavier weight. Beside legs length is directly linked to your speed.$\endgroup$
– RigopOct 13 '16 at 12:34

$\begingroup$The heat loss is another issue I was wondering about. It was brought up here a while ago in a question about fairies out competing humans for survival, but they just handwaved the issue.$\endgroup$
– unknownOct 14 '16 at 2:18

$\begingroup$@Rigop A smaller person is less strong, but they also have less weight to propel (They will not be able to lift the same amount of weight, but they can still jump well). It isn't an exact measurement that jumping will be completely unchanged, but it's a good rule of thumb. Running may be changed a bit more than jumping, because they now "run" with long bounds, but it should come to a comparable speed since running is already partially jumping.$\endgroup$
– IndigoFenixOct 18 '16 at 5:39

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$\begingroup$@AarthewIII When modern humans have dwarfism, it is accompanied by medical conditions, because the dwarfism is generally caused by a genetic defect that may have other effects; also our bodies aren't fully adapted overall to be that size. Humans that belong to a small subspecies shouldn't have those conditions.$\endgroup$
– IndigoFenixOct 18 '16 at 5:42

As far as I can tell it is possible to be less than two feet tall. However, dwarfism is usually due to genetic variance. For example proportionate dwarfism is usually due to organ underdevelopment. Your humans probably should stick above the 4 foot 10 inch bar. (That height being the standard for dwarfism.)

$\begingroup$That's actually the size I was hoping for. Taking them down to less than a foot would seem weird.$\endgroup$
– unknownOct 13 '16 at 6:02

$\begingroup$I'd use "Genetic Variance" rather than "Medical condition", the latter implies that dwarfish can be cured or treated - which is not the case (not that there is anything really wrong with dwarfism though, on a societal level at least).$\endgroup$
– Harry DavidOct 13 '16 at 9:12